This invention relates to a variable magnification, or zoom lens, and more particularly to a reflector zoom lens for projecting an image of an object on a fixed object plane onto a fixed image plane.
The operation of a zoom lens can be thought of as involving two separate functions, though they occur simultaneously. First, the entire lens is generally moved relative to the object and image to change magnification in accordance with the relationship:
Magnification=s'/s PA1 (1/s)+(1/s')=1/f
wherein: s=object distance, s'=image distance
Second, compensation is made within the lens to refocus, or maintain in focus, the lens in accordance with the following relationship
wherein: f=lens focal length. The purpose of refocusing is to maintain s'+s at a constant value for all magnifications.
The compensation in focal length, step two mentioned above, is generally made by movement of one or more of the lens elements within or relative to the lens group.
U.S. Pat. Nos. 3,630,599 and 4,061,419 disclosed representative zoom lens constructions including matched groupings of lenses symmetrically positioned on both sides of a central aperture stop. These lenses are referred to as "full" zoom lenses.
An improved zoom lens which operates with fewer lens elements is disclosed in U.S. Pat. No. 4,056,308. For this type of lens construction, an outer fixed element is arranged symmetrically along the optical axis with a movable lens pair. At the back of the lens is a mirror which folds the optical axis back through the movable and fixed lenses. With this configuration, referred to as a "half" zoom lens, the object and image planes will be on the same side of the half zoom lens. As shown in the '308 patent, at unity magnification, the object and image planes are co-planar; if the lens elements are repositioned for reduction, the image plane will move closer to the lens. For the half zoom lens to be useful in a system where, for example, a document is to be exposed at an image plane located on the other side of the zoom lens, a system of mirrors must be used to fold the image emerging from the half zoom lens back in the original direction of travel.
It would be desirable, therefore to utilize a reflector lens to take advantage of the simplified construction while yet minimizing the requirement for positioning additional folding mirrors in an optical system. The present invention therefore, is directed towards a reflector zoom lens which does not require the use of folding mirrors to redirect a projected image towards an image plane. More particularly, the invention is directed to a variable magnification lens assembly comprising:
a variable magnification lens including the following elements in alignment along the lens optical axis;
a fixed outer lens element, at least one movable lens element spaced from said outer element and movable along said optical axis, and
a reflector spaced from said movable element, said lens further including a first and second reflective element positioned proximate to said fixed outer lens element, said reflective elements joined together to form a right angle, said optical axis intersecting said angle.